The Claus process is widely used to produce sulfur from acid gas and other gases containing hydrogen sulfide. In the modified Claus process feed gas containing hydrogen sulfide is partially combusted with air to form sulfur dioxide. The uncombusted hydrogen sulfide reacts with sulfur dioxide forming sulfur and water in a reaction furnace. The reaction stream is cooled and the sulfur is condensed and recovered. The reaction stream is then passed through one or more catalytic converters wherein additional sulfur is produced in these catalytic stages by the reaction of previously unreacted hydrogen sulfide with sulfur dioxide.
Claus sulfur recovery plants are often employed in petroleum refineries to recover sulfur from waste gas streams as a commercially utilizable product and to reduce air pollution. Sulfur is present in the waste gases in the form of hydrogen sulfide at various concentrations. In the majority of the refineries, the hydrogen sulfide content of the gas is increased by treatment with suitable selective absorbents, such as variously formulated amine based solvents. The principal function of the Claus plant at a petroleum refinery is to convert the hydrogen sulfide content of this more concentrated acid gas stream, which is also referred to as amine gas, to sulfur.
In addition to the acid gas, refineries also have to cope with other environmentally hazardous waste streams. One of such streams is the gas effluent of a sour water stripper. This, so called sour water stripper gas, usually contains ammonia, hydrogen sulfide and water in approximately equal proportions. It is desirable to process such a gas in order to recover the additional sulfur contained therein while simultaneously eliminating an environmentally hazardous by-product. Unfortunately, passing sour water stripper gas in conjunction with acid gas through a Claus sulfur recovery plant present significant operating difficulties.
Major operational difficulties are experienced downstream of the reaction furnace if the ammonia content of the sour water stripper gas is not completely destroyed in the combustion zone of the reaction furnace. Residual ammonia in the Process stream forms undesirable compounds with sulfur, which upon cooling, precipitate solid salts, such as polysulfides of ammonia. The solid salt leads to premature catalyst deactivation in the converters, plugs the process lines and may interfere with the draining of sulfur from the sulfur condenser. Since sulfur vapor is always present in the process stream, the formation of the undesirable ammonia compounds can be prevented only by the upstream destruction of the ammonia.
For an assured destruction of ammonia in a Claus reaction furnace, the thermodynamic and kinetic conditions required for the elimination of this compound would have to be reconciled with the main functional requirement of the reaction furnace, which is to optimize conditions for sulfur production by the Claus reaction. The oxidative destruction of ammonia is favored by high temperatures and high oxygen partial pressures. These requirements cannot be optimized sufficiently in the combustion zone of a conventional Claus reaction furnace if an acid gas is also concurrently combusted in the same combustion zone. The temperature in this combustion zone is constrained by the temperature tolerance of the refractories and the air supply has to be regulated strictly according to the stoichiometric requirement of the Claus reaction, i.e., to combust about one third of the hydrogen sulfide to sulfur dioxide to produce a hydrogen sulfide/sulfur dioxide ratio of about 2. Furthermore, the combustion products of the acid gas and the nitrogen introduced with the supplied air dilute the atmosphere in the combustion zone, reduce the partial Pressure of oxygen for the combustion of the ammonia and can lead to the escape of uncombusted ammonia just by incomplete mixing.
A further obstacle which usually prevents co-Processing sour water stripper gas with acid gas in a Claus sulfur recovery plant is Presented by flow rate restrictions or by limited air blower capacity. The flow rate of the process stream passing through the sulfur recovery plant would increase more than in direct proportion to the additional sour gas input. The increased flow rate can lead to excessive pressure drops and unacceptable pressures in the plant. The unproportional increase in the flow rate is indirectly due to the relatively high oxygen requirement of processing sour water stripper gas, since with each mole of oxygen required for the full combustion of the ammonia and for the partial combustion of the hydrogen sulfide contained therein, about four moles of nitrogen is also introduced into the system from the combustion air. A fully loaded plant, i.e., one which is processing acid gas close to its full flow capacity, cannot recover sulfur from additional sour water stripper gas even if the problems associated with the presence of ammonia in this gas were solved.
It is known in the art that the throughput capacity of a Claus plant can be increased by oxygen enrichment or by the complete replacement of the combustion air with technically pure oxygen. It is also known that an increase in the oxygen concentration of the oxidant increases the flame temperature and, depending on the composition of the feed, may lead to excessive temperatures which can damage the refractories in the reaction furnace. It has been suggested to solve the problem of excessive combustion zone temperatures by recycling a portion of the downstream flow back to the combustion zone to dilute the combustion zone reactants and consequently reduce the combustion temperature. For example, U.S. Pat. No. 3,681,024-Hujsak et al. teaches recycling a portion of the gas effluent from the downstream sulfur condenser(s) to the combustion zone and U.S. Pat. No. 4,552,747 - Goar teaches recycling a portion of the gas effluent from the first sulfur condenser to the combustion zone.
Other temperature moderating additives which have been used or proposed for use in the combustion zone of the Claus reaction furnace include liquid water, liquid sulfur and liquid sulfur dioxide. Temperature moderation is achieved by the absorption of some of the heat released in the combustion zone by the temperature moderating additive.
These temperature moderation processes permit boosting plant capacity by the use of oxygen. However, none of them remove the kinetic constraint imposed on the combustion of ammonia by the operating environment of the combustion zone of a Claus reaction furnace. The combustion temperature is still limited by the furnace refractories and the combustion zone atmosphere is diluted by the combustion products of the acid gas, by a recirculated gas or another temperature moderating additive. Furthermore, a temperature moderating additive introduced into the reaction furnace increases the flow rate through the thermal stage of a Claus plant and, unless the additive is removed from the process stream before the catalytic stages, increases the pressure drop through the entire plant.
Because of these difficulties, ammonia-containing sour gas streams have heretofore generally been disposed of by incineration or by other methods, such as by pumping a concentrated condensate of the gas into the ground, without further processing. This has had the twin disadvantages of the loss of the sulfur contained in such sour gas and the environmental detriment caused by the disposal of hazardous materials. Thus it is very desirable to have a process which would enable the efficient processing of an ammonia-containing sour gas for sulfur recovery in a modified Claus plant.
Accordingly, it is an object of this invention to provide a process wherein an ammonia-containing sour gas may be processed in a modified Claus plant without premature deactivation of the catalyst by ammonia compounds.
It is another object of this invention to provide a process wherein an ammonia-containing sour gas may be processed in a fully loaded Claus plant in addition to acid gas without increasing the pressure drops throughout the plant.
It is a further object of this invention to provide a process wherein an ammonia-containing sour gas may be processed in a modified Claus plant while maintaining a non-excessive temperature in the combustion zone of the Claus reaction furnace.